The East Couloir is best approached from the Mosquito Flat Trailhead (see the Mt. Mills getting there segment for directions to Mosquito Flat). About 1/2 mile from the trailhead, there is a branch to the right, with a sign indicating the way to Mono Pass. Stay on the trail to Ruby Lake, and contour the lake on the east side, and then continue up the drainage that feeds the lake. Stay in the bottom of the drainage, and pass Mills Lake on the left. The couloir will be visible from here.
The route is a straightforward scramble up a steep and sometimes loose couloir, blocked by a large chockstone. The crux of the scramble is passing the chockstone, and in normal dry conditions, this is no problem. Early season there may be ice or snow hindering the ascent, and mid to late season sees the top of the chockstone covered with loose sand. Continue to the top of the couloir, across the summit plateau, and sign in.
Selecting the Correct Chute:
As you reach the top of the approach, you’ll see three chutes. The left (southern) chute appears to have a chockstone, but it is actually the Ranschau/Machleder Variation. This route eventually leads to a class 3-4 headwall with friable handholds and footholds and is not a pleasant alternative. There are many loose rocks and much sand on this route.
The narrower middle chute is also a no-go.
The East Couloir is actually the right (northern) chute.
Also, there are MANY loose rocks on this route. I treaded as lightly as I could and I sent several large rocks careening swiftly down the chute. Thus, to minimize rockfall hazards, this route should be climbed by small parties, especially later in the season when the snow is no longer covering the debris. I strongly recommend that parties wear a helmet.
Secor’s picture on page 328 is somewhat inaccurate. The East Couloir is slightly more to the right than the line indicates.
Additional useful information from Matthew Holliman:
Date: Aug 16, 2004 11:59 PM
I found both the route-finding and climbing to be surprisingly tricky on this peak, and judging from various trip reports I find online, so have many others. I have a few comments to add on selecting the right chute, and passing the chockstone:
If approaching the peak from Mills Lake, the true summit of Mt. Mills is hidden from view by a much more prominent false summit along the north ridge. Furthermore, a sandy chute/ledge system leads up to the ridge north of this false summit. This chute is particularly prominent in early season when a snow finger extends up the gully. As a result, anyone who hasn't seen Mills from the east but who has read Secor's route description may be misled into thinking this gully/ledge system is the East Couloir. It's not! The East Couloir is reached by contouring further south, as depicted in the photo below. (Incidentally, this first gully is the standard--and perhaps only easy--way up to the north ridge from the Mills Lake area; it is ducked and no more than class 2 up to the ridge. The North Ridge itself is a much more solid and enjoyable climb than the East Couloir).
Once you reach the base of the East Couloir, there are two very obvious chutes, as shown in the photo below; the left looks the more promising, and many parties start up there, but it leads to the very loose, dangerous class 4 climbing mentioned by Dave. In my opinion, this lefthand chute is the most dangerous place I've been in the Sierra. The righthand chute is the route mentioned by Secor; it is blocked by a chockstone at the very bottom.
The chockstone is passed on its left. I have to disagree with the comment that passing it is "no problem"--I found it to be harder than it had looked from a distance, and I believe the class 3 rating for the chute stems from the fact that there's little exposure here rather than the difficulty of the climbing. (The ostensibly class 3 section is perhaps only 10ft or so). I climbed it in early August, and found loose rock, poor footholds, and a continuous stream of sand from the slopes above pouring down on my face. Others agreed it wasn't trivial: One register entry thought it to be about 5.2, while another compared it to the Waterfall Pitch on Mt. LeConte. Judging from trip reports and register entries, many parties choose to rappel this on the way down. (For point of reference, I climbed routes such as the North Ridge on Conness, Palisade Crest, and Dragon Peak the same week that I climbed Mills--although short, Mills had the hardest scrambling of any of these peaks). It is likely easier in early season when snow covers both the chockstone and much of the loose rock in the chute.
Once past the chockstone, the couloir is mostly steep, loose class 2, leading eventually to a notch along the north ridge. It is easy to traverse along the west side of the ridge up to the summit plateau. There are several large, loose blocks in the couloir that are waiting to seriously injure/kill someone--extreme caution is needed on this route, and as Dave indicates, I would recommend soloing it only. A safer option is to take the VanDalsem variation mentioned by Secor; this begins some 20-30 yards past the chockstone in a small alcove to the right. There is more class 3 on this variation, but the rock isn't quite as loose (although still not great quality).
In early season, crampons and an axe may be necessary. Once the snow and ice melt, you can climb this thing in just about any footwear you choose. Water is readily available until the last 1500 feet of ascent.